The present invention generally relates to the field of joints for connecting two tubes. More specifically, the present invention relates to connecting two tubes through a ball-cup connection in a high temperature environment.
In high temperature furnaces, tubes made of ceramic materials need to be connected to each other. These tubes may be process tubes, gas-feed tubes, gas-exhaust tubes, thermocouple tubes, and the like. The connection between two tubes is designed such that ingress and egress of gases is prevented.
Ball-cup connections are generally used to prevent ingress and egress of gases. In such a connection, a ball provided on a tube and a mating cup provided on another tube form in a mating and contacting position, wherein the surface-to-surface contact provides a mechanical seal. Typically, a ball and cup are made of the same material, and the design is such that the tolerances in the sizing of the ball and the cup can be accommodated. When the ball and cup are made of different materials, the design needs to be such that differences in the thermal expansion of the two materials can be accommodated. In such cases, the ball and cup exactly fit together at room temperature, but when heated, the actual contact between the ball and cup reduces from an area contact to a line contact, due to differences in the thermal expansion of the materials. Since the diffusion of gases occurs over the line contact easily, the ball-cup connection is not perfect. To minimize the diffusion, a clamp is used to press the ball against the cup, and an O-ring is required at the ball-cup interface, to provide tight sealing for the gas. However, at very high temperatures, the O-ring may degenerate and deteriorate the quality of the connection.
In U.S. Pat. No. 6,499,768, co-assigned with the present application, a ball-cup connection with a gas seal to prevent ingress and egress of gases is disclosed. In the '768 patent, a first tube is provided with a ball interface, which is located at an end of the first tube. Further, a second tube is provided with a cup interface, which is located at an end of the second tube. The cup interface of the second tube mates with the ball interface of the first tube and forms a cup-ball interface for connecting the first and the second tube, wherein the first and the second tube are external to each other. However, in some situations, there is a need to connect two tubes wherein one of the tubes is located inside the other tube. A connection known in the prior art that meets this need is an ultratorr connection, which is used in the vacuum technology. An ultratorr connection uses an O-ring that is compressed and sealed against the outside diameter of a tube that needs to be a vacuum tight connection to a vacuum chamber. However, due to the use of the O-ring, the ultratorr connection is not resistant to high temperatures.
Accordingly, there is a need for a connection that connects two tubes, wherein one tube is inside the other and the connection is resistant to high temperatures. Moreover, the connection should prevent ingress and egress of gases.